Various methods for making perforated polymeric films are known. Generally the perforated polymeric films previously have been fabricated with either a high percent open area or a high thickness to hole spacing ratio, but not both. The percent open area is a relative measure of how much filter media area is needed to accommodate a flow rate. Generally, a larger percent open area is preferred because it can accommodate a greater flow rate in a smaller area. The ratio of film thickness to the shortest distance between adjacent holes is a relative measure of the physical strength of the microperforated polymeric film and the perforation density. A larger ratio is preferred because it indicates superior strength and durability while having closely located holes.
For example, G.B. Pat. No 851,473 generally describes the process of perforating a thermoplastic polymeric material by melting the material at selected localized areas, while simultaneously cooling other areas of the polymeric material to prevent them from melting. Using a ¼ mil (6.4 micrometers) polymer film, holes 33 mils (838 micrometers) in diameter were able to be fabricated with approximately 237 holes per square inch, equating to a ratio of film thickness to the shortest distance between adjacent holes of 0.007 and an open area of 20%. Using a 2 mil (50.8 micrometers) polymer film, holes 250 mils (6350 micrometers) in diameter were able to be fabricated with approximately 8 holes per square inch, equating to a ratio of film thickness to the shortest distance between adjacent holes of 0.015 and an open area of 39%.
G.B. Pat. No. 1,073,605 (Rowley) describes the process of perforating thin thermoplastic films containing recesses. The films, recess-side down are passed around a chilled cylinder while exposing the other side to the heat of a flame. The chilled cylinder cools the film, as the heat selectively melts the bottoms of the recesses, perforating the film. This patent discloses polymeric film thickness of 0.100 inches (2540 micrometers) and 400 recesses per square inch having a diameter of 1/64 of an inch (396 micrometers), equating to a ratio of film thickness to the shortest distance between adjacent holes of 2.6 and an open area of 8%.
U.S. Pat. No. 3,560,601 (Johnson, et al.) describes the process of perforating a fabric backed polyvinyl chloride material by first embossing the material to create depressions in the material and then contacting the material with heated air to remove the thin sections remaining at the bottoms of the depressions.